The present invention relates to a signal combining circuit having the first inversion circuit that inverts an analog input signal, the second inversion circuit that inverts the analog signal inverted by the first inversion circuit, an A/D converter that inputs output signals from the first and second inversion circuits and converts the signals to digital signals, the third inversion circuit that inverts another analog input signal, the fourth inversion circuit that inverts the other analog input signal inverted by the third inversion circuit, another A/D converter that inputs output signals from the third and fourth inversion circuits and converts the signals to other digital signals, and a digital mixer circuit that combines the digital output signals from the A/D converter and the other digital output signals from the other A/D converter.
To convert two analog signals, e.g., a synchronization signal and a video signal to digital signals, respectively, and then to combine the digital signals, there has been conventionally employed a signal combining circuit having two A/D converters and a digital mixer circuit as described in the introduction part. FIG. 2 is a circuit block diagram of a conventional signal combining circuit having two A/D converters and a digital mixer. An analog input signal from an input terminal 1 is inverted by the first inversion circuit 3 and inputted into the negative input of an A/D converter ADC1. The inverted input signal is further inverted by the second inversion circuit 4 and inputted into the positive input of the A/D converter ADC1. The A/D converter ADC1 converts these difference input signals to digital signals. After the magnitudes of the signals converted into digital signals are adjusted by a digital volume 8, the digital signals are inputted into a digital mixer circuit 10. Likewise, another analog input signal from an input terminal 2 is converted to other digital signals by the third and fourth inversion circuits 6, 7 and another A/D converter ADC2, the magnitudes of the signals are adjusted by a digital volume 9 and then the digital signals are inputted into the digital mixer circuit 10. The digital mixer circuit 10 combines these two digital input signals to thereby output one digital signal.
The digital combining signal circuit having these two A/D converters can be also utilized when one analog input signal is converted to one digital signal. In that case, one of the A/D converters may be used to convert the analog signal to the digital signal and the input of the other A/D converter may be set at 0V.
Meanwhile, it is ideal that these A/D converters have linearity, i.e., the level of a digital output signal is proportional to that of an analog input signal. However, there are cases where an actual A/D converter has nonlinearity. For example, if an analog signal of 1V is inputted into an A/D converter, an output signal from the A/D converter should be a digital output signal corresponding to original level of 1V, but a digital output signal corresponding to 0.8V level is sometimes outputted. If the A/D converters are the same in type or these A/D converters are included in one IC, they tend to have same nonlinearity.
The inventor of the present invention contrived a digital signal combining circuit which employs two A/D converters to convert one analog signal to one digital signal and compensates for the distortion of the output signal due to the nonlinearity of these two A/D converters. Further, it was discovered that this circuit can obtain an effect of improving S/N ratio.